Converting .3mf to .blend
At the time of this writing (April 2017), it appears that this might need to make use of an external platform like Spin 3d from NCH Software; there does not presently seem to be a way to do it directly. However, that said, the 3MF format specification is open source, and I would expect that there will be, sooner or later, the possibility of direct import to, and direct export from, Blender. 

Implementing either of these will take programming capabilities that are outside of my skillset, so I won't have direct involvement.